1. Field of the Invention
The present invention relates to universal joints, and more particularly to a rotary coupling capable of transmitting even very large driving forces from a driving shaft to a driven shaft, at any angle between the two shafts from 0.degree. up to and including 90.degree..
2. Description of the Prior Art
Rotary couplings of the crown gear type are well known, in which each of a driving and driven shaft are provided with a crown of gear teeth. When the shafts are aligned, the crowns on the respective shafts intermesh axially; when the shafts are angularly offset, at least one of the gear teeth of one crown is always engaged between a pair of teeth on the other crown. In this manner, power may be transmitted from the driving shaft to the driven shaft from a position of alignment of the two shafts up to a maximum angular displacement of the two shafts. Couplings of this type are particularly prevalent in dental tools.
An example of a prior art rotary coupling as described above is found in U.S. Pat. No. 1,241,118, to Hoskins. This Hoskins patent, like much of the prior art, mounts the gears in a hinged casing which serves to maintain the teeth of each gear in correct position relative to one another. Other examples of hinged casings used to correctly position the gear clusters of axially intermeshing crown gear couplings are provided in U.S. Pat. Nos. 636,476; 2,316,243, and Reissue 22,555.
The closest known prior art to the present invention is U.S. Pat. No. 4,789,377, in the name of Nathan D. Hoskins, which solves a number of the problems associated with the prior art described above, by the provision of a slotted casing containing a pair of crown gear clusters having semi-spherical gear teeth and semi-spherical sockets between adjacent gear teeth. As the present invention represents an improvement on, as well as a departure from, the universal joint described in U.S. Pat. No. 4,789,377, the entirety of that patent is hereby expressly incorporated by reference.
It has been found by the present inventors that the universal joint of U.S. Pat. No. 4,789,377 is nevertheless attended with several disadvantages that restrict its commercial feasibility. In particular, it has been found that the semi-spherical shape of the gear teeth on the crown gear clusters is very difficult to machine, which causes the manufacture of such a device to be excessively complicated and expensive.
Moreover, the slotted casing of this prior art universal joint is provided with differentiated bearing surfaces 14, 15 for the driven gear, which bearing surfaces 14, 15 differ substantially in height along the longitudinal dimension of the casing. Still further, the device described in this patent requires the use of an undulate spring 23 captive on the shaft of the angularly movable driven gear, which is thus interposed between the base of the crown gear cluster and the differentiated bearing surfaces 14, 15. In manufacturing devices according to the teachings of U.S. Pat. No. 4,789,377, it has been found that the presence of spring 23, although necessary to urge the gear clusters of the drive and driven shafts into meshing engagement, nevertheless detracts from the smooth operation of the device, particularly when the driven gear 1 is moved through its 90.degree. range of travel and the spring 23 is thereby caused to slide across the differentiated bearing surfaces 14, 15.
An additional disadvantage of the universal joint described in U.S. Pat. No. 4,789,377 is that the structure of the slotted casing has proven to be undesirably large and insufficiently sturdy for especially heavy duty applications.